H. B. A. Prins

LIGHT-INDUCED MEMBRANE-POTENTIAL CHANGES OF EPIDERMAL AND MESOPHYLL-CELLS IN GROWING LEAVES OF PISUM-SATIVUM

Light transiently depolarizes the membrane of growing leaf cells. The ionic basis for changes in cell membrane electrical potentials in response to light has been determined separately for growing epidermal and mesophyll cells of the argenteum mutant of pea (Pisum sativum L.). In mesophyll cells light induces a large, transient depolarization that depends on the external Cl− concentration, is unaffected by changes in the external Ca2+ or K+ concentration, is stimulated by K+-channel blockers tetraethylammonium (TEA+) and Ba2+, and is inhibited by 3-(3′-4′-dichlorophenyl)-1,1-dimethylurea (DCMU). In isolated epidermal tissue, light induces a small, transient depolarization followed by a hyperpolarization of the membrane potential. The depolarization is enhanced by increasing the external Ca2+ concentration and by addition of Ba2+, and is not sensitive to DCMU. Epidermal cells in contact with mesophyll display a depolarization resembling the response of the underlying mesophyll cells. The light-induced depolarization in mesophyll cells seems to be mediated by an increased efflux of Cl− while the membrane-potential changes in epidermal strips reflect changes in the fluxes of Ca2+ and in the activity of the proton-pumping ATPase.

Bi-phasic composition of trans-root electrical potential in roots of Plantago species: involvement of spatially separated electrogenic pumps.

The effect of oxygen on the trans-root potential (TRP) of excised roots in Plantago media L. and P. maritima L. was investigated. Two distinct reactions were found. In some experiments (type A roots) the reaction of TRP to anoxia was bi-phasic, and this reaction fits well into a model, assuming the presence of two spatially separated proton pump sites in the roots: one at the plasmalemma of epidermal and cortical cells and the other at the symplast/xylem interface. The two pumps work in opposite directions. In other experiments (type B roots) no hyperpolarization as a response to anoxia at the inner symplast membrane was observed. There is evidence that the inner pump is also present in these roots, but only in an inactive or electroneutral state. It is concluded that O2-deficiency prevails more often in the central part of the root than in epidermal and cortical cells, when roots are brought gradually under anoxia. This causes the pump located at the symplast/xylem interface to be inhibited more quickly than the other at decreasing O2-concentrations in the bathing solution.

A CO2-flux mechanism operating via pH-polarity in Hydrilla verticillata leaves with C-3 and C-4 photosynthesis

The aquatic angiosperm Hydrilla verticillata lacks Kranz anatomy, but has an inducible, C4-based, CO2 concentrating mechanism (CCM) that concentrates CO2 in the chloroplasts. Both C3 and C4Hydrilla leaves showed light-dependent pH polarity that was suppressed by high dissolved inorganic carbon (DIC). At low DIC (0.25 mol m−3), pH values in the unstirred water layer on the abaxial and adaxial sides of the leaf were 4.2 and10.3, respectively. Abaxial apoplastic acidification served as a CO2 flux mechanism (CFM), making HCO3− available for photosynthesis by conversion to CO2. DIC at 10 mol m−3 completely suppressed acidification and alkalization. The data, along with previous results, indicated that inhibition was specific to DIC, and not a buffer effect. Acidification and alkalization did not necessarily show 1:1 stoichiometry; their kinetics for the apolar induction phase differed, and alkalization was less inhibited by 2.5 mol m−3 DIC. At low irradiance (50 μmol photons m−2 s−1), where CCM activity in C4 leaves is minimized, both leaf types had similar DIC inhibition of pH polarity. However, as irradiance increased, DIC inhibition of C3 leaves decreased. In C4 leaves the CFM and CCM seemed to compete for photosynthetic ATP and/or reducing power. The CFM may require less, as at low irradiance it still operated maximally, if [DIC] was low. Iodoacetamide (IA), which inhibits CO2 fixation in Hydrilla, also suppressed acidification and alkalization, especially in C4 leaves. IA does not inhibit the C4 CCM, which suggests that the CFM and CCM can operate independently. It has been hypothesized that irradiance and DIC regulate pH polarity by altering the chloroplastic [DIC], which effects the chloroplast redox state and subsequently redox regulation of a plasma-membrane H+-ATPase. The results lend partial support to a down-regulatory role for high chloroplastic [DIC], but do not exclude other sites of DIC action. IA inhibition of pH polarity seems inconsistent with the chloroplast NADPH/NADP+ ratio being the redox transducer. The possibility that malate and oxaloacetate shuttling plays a role in CFM regulation requires further investigation.

The influence of pH on ectomycorrhizal development of Pseudotsuga menziesii inoculated with Laccaria bicolor in hydroculture.

Abstract It was possible to grow healthy mycorrhizal Pseudotsuga menziesii seedlings on a medium with defined ionic ratios and a pH of 4. However using a slightly different solution (lower in phosphate) resulted in an acidification of the medium below pH 3.5 and this was fatal for mycorrhiza formation.

A study of the electrophysiological organization in roots of two Plantago species: direct measurement of ion transport to the xylem using excised roots

Regulation of sodium transport across the plasmalemma of root cortical cells differs greatly between halophytes and glycophytes. Halophytes are known to accumulate salts6 whereas glycophytes respond to salinity basically by ion exclusion8. The mechanism which enables glycophytes to exclude Na+ ions is probably a coupling between K+ uptake and Na+ extrusion; this is indirect and a proton motive force (p. m. f.) generated by an electrogenic proton pump is thought to power this exchange10.